Lithography is widely recognized as one of the key steps in the manufacture of integrated circuits (ICs) and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of ICs. In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In lithographic apparatuses typically many moving parts are provided with various degrees of freedom, and the positions (including linear and angular position (orientation), velocities and accelerations) are controlled automatically via numerous actuation mechanisms (actuators). Actuators may be electromagnetically operated, pneumatically of hydraulically operated. They are often constrained to effect movement in only one degree of freedom (linear or rotational). Where the moving parts are to be controlled in plural degrees of freedom, more complex mechanisms may be provided, or multiple single-degree mechanisms may be combined.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation sources are typically configured to output a radiation wavelengths of around 5-20 nm, for example, 13.5 nm or about 13 nm or 6.5-6.8 nm. Thus, EUV radiation sources may constitute a significant step toward achieving small features printing. Such radiation is termed extreme ultraviolet or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. Because of the need for extreme accuracy, and because additionally of the need to work a vacuum environment with high reliability, designing actuators for EUV lithography apparatus is particularly demanding.
An example where arrays of actuators are required is in the facetted mirrors of an illumination system of an EUV optical apparatus. Numerous individual mirror facets may be provided in an array, each of which may need to be oriented in different directions to effect different illumination profiles at a target location. Actuators for field facet mirrors are described for example in the published international patent application WO 2011/000671 A1. The mechanisms described in that publication provide only two positions for each facet mirror, which are conveniently set by end stops. When seeking to extend the range of illumination profiles that can be achieved, actuators having more than two positions are desired, which may include movements in two or more degrees of freedom and may require intermediate positions that cannot be defined by end stops. Providing robust support for the field facet mirror in that case does not allow the mounting to have lower stiffness in one direction and higher stiffness in another. Therefore to provide a robust mounting implies an increase in the force required of the actuator motor. Whatever form the actuator takes, it should meet stringent requirements of size, cost and heat dissipation, as well as performance.